The past week has been filled with "Guardians of the Galaxy" casting rumors, but perhaps none were as tantalizing as the possibility of Joseph Gordon-Levitt playing Star-Lord. That possibility has now apparently been put to bed, but it's worth noting that while Gordon-Levitt may have been in the running to play the Guardians' lone human member, he first gained fame playing an alien on the critically acclaimed sitcom "3rd Rock From The Sun."

Given that, this week we’ve decided to put the spotlight on three of the most notable alien races of the Marvel Universe, some of whom may well play a role in upcoming Marvel films.

The Kree

A militaristic empire dedicated to expanding their influence across the universe, the Kree have been meddling in human affairs for countless millennia, even using their expertise in genetic engineering to create the offshoot of humanity known as the Inhumans. Ruled by the Supreme Intelligence, an entity composed of the greatest minds in Kree history, the Kree have watched our world develop, waiting for the right moment to claim it as their own, but things haven't always exactly gone according to plan.

Sent to infiltrate human society in advance of a full-fledged invasion, the Kree soldier Mar-Vell came to sympathize with humanity, eventually becoming the hero known as Captain Marvel and turning against the warlike ways of his people. Mar-Vell saw the destructive ends their path would take, and indeed the Kree empire has been devastated by countless wars, first against their ancient enemies the Skrulls, and later against the Shi'ar, when a deadly weapon known as the Nega-Bomb decimated most of the worlds in their galaxy.

Most horrifying of all, it was found that the Supreme Intelligence had manipulated this course of events in order to jumpstart what he saw as stagnant Kree evolution. But those twisted plans for Kree supremacy came to an ironic pass, when after years of war and destruction, the once-mighty Kree Empire was left vulnerable to a takeover by the very beings they had created—the Inhumans. The Supreme Intelligence has since reclaimed control of the remnants of his empire, but it remains to be seen whether, with all the minds at his disposal, he has learned anything from the devastation his schemes have inflicted on his people.

The Skrulls

A malevolent race of alien shapeshifters, the Skrulls too have sought to conquer Earth, though their methods tend to be a bit less direct than those of their hated rivals, the Kree.

Able to take any form they choose, the Skrulls first tangled with the Fantastic Four, impersonating the team with the goal of discrediting them, and weakening Earth for an impending invasion. That scheme ended in the infiltrators submitting to hypnosis and living out the rest of their lives in the form of cows, but the Skrull Empire was not through with Earth. They empowered one of their champions, Kl'rt, with the combined powers of the Fantastic Four, transforming him into a Super-Skrull, though he too failed to defeat the real deal. One of their agents, a female Skrull by the name of Lyja, went so far as to take the place of the Thing's girlfriend, Alicia Masters, and married Johnny Storm before her deception was uncovered.

Most recently, in the wake of their homeworld being destroyed during the "Annihilation" storyline, the Skrulls followed a prophecy that saw them launch a massive "Secret Invasion" of Earth. The initial phase involved kidnapping and replacing many of Earth's greatest heroes, before commencing a full-scale attack with hundreds of super-powered Skrulls, who wreaked global havoc before finally being defeated. Few among Earth's superhuman community believe they've seen the last of the Skrulls, however, and they continue to watch those around them closely for signs of Skrullish behavior.

The Shi'ar

Sometimes foes, and sometimes allies of the team of mutants known as the X-Men, the path of the Shi'ar Empire has always depended on who is ruling it.

When the X-Men first encountered them, they were under control of the mad emperor D'Ken, who sought to use the reality-bending M'Kraan Crystal to expand his power. With the aid of his more rational sister, Lilandra, the X-Men were able to thwart D'Ken's plans, and Lilandra, now empress, struck up a romance with the X-Men's leader, Charles Xavier. Lilandra took her new responsibilities seriously however, and when the power of the Phoenix began to corrupt Jean Grey and threaten all life in the universe, the empress of the Shi'ar confronted the threat head on, seeing it necessary to eliminate Jean, even if it meant wiping out our entire solar system — a fact Jean herself acknowledged, when she took her own life to avert that catastrophe.

Lilandra's rule would not go unchallenged for long, however — the consequences of D'Ken's mad reign came back to haunt her in the form of the man known as Vulcan. The long-lost brother of X-Men Cyclops and Havok, Gabriel Summers was born and raised in the Shi'ar Empire, after D'Ken kidnapped his father and pregnant mother. Gabriel was enslaved and abused, warping his mind, and leading him to seek vengeance on the Shi'ar Empire years later, under the name Vulcan. He ultimately achieved his goal by overthrowing Lilandra, and taking the role of emperor for himself, but his madness led him into conflict with the Kree Empire and their new leader Black Bolt, the powerful king of the Inhumans. Vulcan was seemingly destroyed in the ensuing conflict, and Gladiator, the leader of the Shi'ar Imperial Guard, assumed the throne reluctantly, agreeing to serve as caretaker for the victorious Kree. But a galactic empire with such a long and proud history surely will not be content to remain under the thumb of a conqueror for long.

Our weekly Galaxy Guide takes you through the cosmic side of comic book movies, from "Guardians of the Galaxy" to infinity and beyond. Tell us what you think in the comments section or on Twitter!